Microirrigation: Soil Moisture, Cover Crop, Irrigation Requirements
From the May 2008 article "Micro Matters: Microirrigation can be an effective method of conserving water, as long as you follow the rules."
By Farouk A. Hassan, Ph.D.
The competitive demand for water led to increasing emphasis on conservative utilization of water used for crop production. Irrigation scheduling is an integrated part of any sound water management and conservation program because it is essential for efficient utilization of water for crop production and for maintaining the profitability of farming operations. It is also essential for preserving water resources and for protecting the environment.
| Moisture Characteristics Of Soils Of Different Texture |
Available Water Tensiometer |
Reading (centibar) at Field Capacity Field Capacity 20-25% |
Soil Texture (inches per foot) Depletion |
| Sand |
0.50 - 1.00 |
10 - 15 |
20 - 25 |
| Loam |
1.00 - 1.75 |
15 - 20 |
20 - 30 |
| Clay |
1.75 - 2.25 |
20 - 25 |
25 - 35 |
Irrigation Scheduling For Orchards And Vineyards
Perennial crops, such as trees and vines, take several years to reach maturity. They also go through several stages of growth and development each season. Immature crops use less water than fully grown trees. The value of the crop coefficient, Kc, for different crops varies with both the degree of maturity and the seasonal stage of growth. Crop coefficients for orchards and vineyards are discussed below.
1. Seasonal Development Coefficients
The crop coefficient, Kc, for mature orchard or vineyard is estimated by recognizing separate stages of growth and development during the
cropping season. Each stage of growth is assigned a separate crop coefficient as follows:
- The Season: is the period of time from leafout to leaf dropping
- Rapid Growth Stage: assigned the crop coefficient "Kc1" from leafout to when leaves grow to near their maximum size and vegetative growth has slowed.
- Midseason Stage: assigned the crop coefficient "Kc2" from end of rapid growth to when the crop leaves aging begins. The end of this stage could be difficult to determine. However, it may be estimated at about 80% to 85% of the season from leafout.
- Late Season Stage: assigned the crop coefficient "Kc3" from beginning of leaf aging to leaf dropping.
The table below gives the values of Kc1, Kc2, and Kc3 for orchards and vineyards in the central valley of California. These values may be adjusted upward for the desert valleys and downward for the coastal valleys according to local experience.
Development coefficients for orchards and vineyards
| Crop |
Kc1 |
Kc2 |
Kc3 |
| Deciduous orchards |
0.5 |
1.0 |
0.7 |
| Grapes |
0.3 |
0.8 |
0.3 |
| Olives |
0.5 |
0.8 |
0.5 |
| Pistachios |
0.3 |
1.2 |
0.3 |
| Citrus |
0.7 |
0.7 |
0.7 |
2. Cover Crop Coefficient
The above listed crop coefficients are for fields without cover crop (i.e. clean cultivation). Orchards and vineyards with active cover crop consume more water and crop factors are adjusted upward by multiplying them by a factor of 1.3. This coefficient will be referred to as the "cover crop coefficient." A cover crop coefficient equals one is used for clean cultivated fields.
For mature orchards or vineyards, with cover crop:
- ET (inch) = development coefficient x cover crop coefficient x ETo
- Water use rate (gallons per tree per day) = ET (inch) x tree spacing (ft) x row spacing (ft) x 0.623
- Irrigation requirements (gallons per tree per day) = water use rate ÷ irrigation efficiency
3. Canopy (Maturity) Coefficient
The degree of maturity is expressed subjectively as the percentage of ground shaded by the crop canopy at midday during midseason. Cover crop shading is not included in estimates of shading by orchard or vineyard. The factor accounting for the percentage of ground shading will be referred to as the "canopy coefficient." It is used to modify the crop coefficient, Kc, for the degree of maturity of the tree or vine. The table below gives the percentages of ground shade (or maturity) and the corresponding values of canopy coefficient for deciduous orchards, citrus and vineyards:
Percentages of ground shade and canopy coefficients for orchards and vineyards:
| Ground Shaded (%) |
10 |
25 |
50 |
60+ |
| Canopy coefficient |
0.3 |
0.6 |
0.9 |
1.0 |
For immature orchards and vineyards, with cover crop:
- ET (inch) = development coefficient x cover crop coefficient x canopy coefficient x ETo
- Water use rate (gallons per tree per day) = ET (inch) x trees spacing (ft) x rows spacing (ft) x 0.623
- Irrigation requirements (gallons per tree per day) = water use rate ÷ irrigation efficiency
- Estimated irrigation Time (hours) = [irrigation requirements (inches) x acres x 452.5] ÷ system flow rate where:
- System flow rate = average flowmeter reading in gallons per minute.
- Acres = number of acres in the irrigated field
Irrigation time can also be estimated using the tree spacing and the discharge rate of emitter(s) per tree as follows:
- Estimated irrigation time, hours = [0.623 x St x Sr x Irrigation Requirements (inches)] ÷ q
where:
St = trees spacing (ft)
Sr = rows spacing (ft)
q = total emitters discharge per tree (gallon per hour)
However, the totalized readings of the flow meter should always be referenced before terminating the irrigation run to ensure that irrigation requirements of the field have actually been applied.
Farouk A. Hassan is an irrigation and soils consultant with Agro Industrial Management in Fresno, CA; e-mail fahassan@aol.com.
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